Abstract A series of acid mordenites having SiO 2 Al 2O 3 ratios from 12 to 97 was prepared for a study of their physical and catalytic properties. The removal of alumina by acid extraction results in a shrinkage in the unit cell. This is indicated by an increase in the frequency of the infrared lattice vibrations and a shift in the position of the X-ray diffraction lines. Infrared spectra show OH groups absorbing at 3740 and 3590 cm −1. The latter are no longer present on severely acid extracted mordenites. NH 3 adsorption isobars at 8 mm show that the surface acidity decreases in a regular manner with an increase in SiO 2 Al 2O 3 ratio. At the same time, acid treatment opens up more of the channels to permit the adsorption of Decalin. The mordenite becomes less resistant to the adsorptive diffusion of these large molecules. Catalysts containing 0.5% Pd were tested for the hydroisomerization of n-pentane. In this reaction, surface acidity appears to be the dominant factor and the catalyst activity decreases with an increase in SiO 2 Al 2O 3 ratio of the mordenite.